1. Field of the Invention
This invention relates to DC-to-DC power converters, and more particularly to resonant power converters of the series-parallel type.
2. Description of the Prior Art
Various DC-to-DC power converters are available for transforming an input DC voltage of one magnitude to an output DC voltage with a different magnitude. Two conventional converter topologies are referred to as the flyback and the forward converters. They are discussed, for example, in a text by George Chryssis, "High-Frequency Switching Power Supplies: Theory and Design", Mc-Graw-Hill Book Company, 1984, pages 11-13.
With a flyback converter, a switch is connected in series with the input winding of a transformer. The switch is alternately turned on and off, producing a pulsing in the secondary winding which is fed through a diode to charge an output capacitor. When the primary current is switched off, the current in the secondary tends to surge. The rates of change of both the primary and secondary currents are very high, leading to electromagnetic interference and radio frequency interference. Complex filters are required to suppress the interference, thereby increasing the complexity and cost of the system and reducing its efficiency.
In the forward converter design an inductor is added to the secondary circuit to reduce the absolute current magnitude in the secondary, while a second diode in the secondary circuit closes a circuit between the output capacitor and inductor when the input switch is off. This design uses a high input current, which is stressful for the switching transistor in the primary circuit. The output diode is stressed by a large voltage in the current swings, requiring a snubber circuit which adds to the cost and complexity of the system and is an interference source. The large rates of current change in the transformer windings and in the inductor produce electromagnetic and radio frequency interference, which again require complex filters to remove.
Many of the problems associated with flyback and forward converter designs are resolved by the more recent "resonant" converter, which is exemplified in U.S. Pat. No. 4,415,959 to Vinciarelli. In this type of device, the most relevant embodiment of which is shown in FIG. 4 of the patent, a relatively large inductor acts as a current sink in the secondary circuit. A capacitor in the secondary circuit cooperates with the leakage inductance of the transformer to establish an effective LC circuit; this defines a characteristic time scale for the rise and fall of current from the DC voltage source. A switch device in the primary circuit can thus be switched on and off at essentially zero current, thereby overcoming the problems in both the flyback and forward converters associated with switching under high current levels. Following each cycle the energy stored in the capacitor is released by the current sink. After the capacitor has been discharged, the sink current is carried by a diode connected in parallel with the capacitor.
There are three basic types of resonant converters which may be used for high frequency switching power supply applications, namely, the series, parallel and combination series-parallel types. In a typical series type resonant converter, one or more resonant capacitors is in series with the primary inductor or primary winding of a transformer in the resonating LC tank circuit. In the parallel converter type, the resonant capacitor is in parallel with the primary inductor or primary winding to form the tank circuit; any input capacitors serve only to split the input DC voltage. The series-parallel resonant converter has both series resonant and parallel resonant capacitors. A detailed analysis of characteristics of each of these three types of resonant converters is found, for example, in the paper by R. L. Steigerwald entitled "A Comparison of Half-Bridge Resonant Converter Topologies", presented at the Second Annual IEEE Applied Power Electronics Conference and Exposition, Mar. 2-6, 1987.
Each of these resonant converter types has certain advantages and disadvantages. A main advantage of the series resonant converter is that conduction and other losses decrease at lighter loads, thereby achieving a high efficiency over a wide load range. In the series converter the load current is proportional to the current flowing through the resonant tank circuit components and the switching devices. A full bridge implementation of the series resonant converter may be made for high power applications without complicated circuitry since the resonant series capacitor prevents DC voltage from building up across the isolation transformer.
A primary disadvantage of the series resonant converter is that the output cannot be regulated at no-load conditions over the practical frequency range for the preferred above- resonant frequency operation. The output filter capacitor has to be large to carry a high value of ripple current since the ripple current is equal to 48% of the output current. This large capacitor conflicts with the objective of size reduction in high frequency power supplies for modern applications. Also, a series resonant converter may not be desirable for applications having severe short-circuit and high output current requirements. A short-circuit at the output results in a very high, undesirable switch current unless the switching frequency is raised sufficiently.
One advantage of the parallel resonant converter is that it can regulate its output voltage at no-load conditions by raising the switching frequency. Although a parallel resonant converter needs an inductor and a capacitor for filtering, the overall filter size is much smaller than that of a series resonant converter because the capacitor carries a low ripple current.
A disadvantage of the parallel resonant converter is that the magnitudes of the currents in the switches and resonant tank circuit components are relatively independent of the load. Moreover, these currents increase as the input voltage increases. Efficiency of the converter is thus diminished at light load or high line voltage conditions because the power loss stays the same or even increases while the output power decreases. A parallel resonant converter is therefore more suitable when the load or line voltage varies over a narrow range.
The combination series-parallel resonant converter has the advantages of both the series and parallel resonant converters without their disadvantages if the resonant components are properly chosen. It has been found that when the series resonant capacitor equals the parallel resonant capacitor, the efficiency remains constant over a wide load range, and the output current can be regulated at no-load conditions with a reasonable upper frequency. A series-parallel resonant converter with the series and parallel capacitors being equal takes on the characteristics of a series resonant converter while operating or departing from full load. The power loss decreases with load by several orders of magnitude before it stops decreasing as the parallel resonant characteristic of the converter takes over to keep the output regulated at no-load conditions. These and other features of the series-parallel resonant converter are found in a paper by K. D. T. Ngo and R. L. Steigerwald et al. entitled "A High-Density Power Supply Using High Voltage IC", presented at the 1987 High Frequency Power Conversion International Conference in Washington, D.C., April, 1987.
To reduce the size of power supplies intended for use in modern systems, it is desirable to raise the operating frequency of the converter. This in turn reduces the size of the reactive components, resulting in an overall size reduction of the converter. However, the increase of switching frequencies has a tendency to increase electromagnetic interference emissions from the converter, requiring additional filter circuitry to remove the emissions. It is therefore desirable to reduce the size of the reactive components in a high frequency converter while at the same time reducing the emission of electromagnetic interference signals.
U.S. Pat. No. 4,791,542, issued Dec. 13, 1988, to A. Piaskowski, entitled "FERRORESONANT POWER SUPPLY AND METHOD", discloses a resonant power converter including a transformer provided with bias windings which enable modulation of the primary inductance of the transformer by controlled saturation of its core. The converter includes switching transistors driven by an oscillator which generates a fixed frequency signal under all operating conditions of the device.
Another pertinent reference is found in IBM Technical Disclosure Bulletin, vol. 27, no. 9, (February 1985) to E. Dobberstein, entitled "VERY HIGH FREQUENCY FM-REGULATED POWER SUPPLY WITH ENHANCED OUTPUT POWER CAPABILITY". This device includes a series resonant circuit, an output of which is inductively coupled to a rectifier stage by a transformer. The transformer primary winding has a value of inductance which is negligible compared to the value of the series inductor, and therefore does not produce a significant frequency shift in the series resonant circuit. The power supply offers enhanced power output capability by placing capacitors across the switching devices of the supply, thereby greatly reducing the device power dissipation during the turn-off transient of the switching cycle.